Stoker grate



Jan. 10, 1951 E. R. MITCHELL ETAL 2,967,496

sToKER GRATE Filed Nov. 18, 1959 2 52 INVENTOR U21/C? Fran Fria ATTORNEY United States Patentv Oiice 2,967,496 Patented Jan. Vl0, 1961 STOKER GRATE Earland R. Mitchell, 32 Bullock Ave., and Frank D. Friedrich, 135 Northwestern Ave., both of Ottawa, Ontario, Canada Filed Nov. 18, 1959, Ser. No. 853,802

Claims. (Cl. 110-38) This invention relates to Stoker grates. A conventional stoker comprises a steam-driven ram feeding coal into a central retort, in which the coal piles up and spills over onto the grate. The grate comprises a plurality of alternately fixed and moving bars placed at right angles to the retort. The moving bars have a slight reciprocating motion which feeds the fuel across the grate to the dump trays. The grate bars are frequently hollow grey iron castings designed so that air for combustion and cooling passes from a main air chamber through them to auxiliary air chambers. From there it passes upwardlybetween the grate bars through the fuel bed thereon.

While with certain types of coal, such a stoker operates satisfactorily, severe operating and maintenance problems are encountered when burning certain other types of coal. For instance, when Eastern Canadian coals are employed, because the ash fusion temperature is low, slag soon plugs the air openings in the bars, obstructing air ow and reducing burning rate'. lThis causes excessive caking, and large cakes of burning coal tumble off the grate onto the dump trays. This results not only in poor burnout and reduced eliiciency but in severe overheating of the tails of the grate bars, so that they soon burn through. When this happens, air distribution is badly upset. Air pours out of the tails and over the dump trays, forming long torchlike llames that damage the brickwork of the furnace side walls and accelerate burning of the bars. Consequently, even less air becomes available at the center of the grate so that combustion conditions further deteriorate. This in turn subjects the bars and furnace walls to even worse overheating, and a grate seldom lasts more than one heating season. Because of their complicated shape, the bars are expensive to manufacture, and maintenance costs are accordingly high.

Efforts have been made to overcome the problems set forth above, but no complete solution of these problems has heretofore been achieved.

An object of the present invention is to provide a grate which is relatively simple in structure and inexpensive to manufacture and which will result in successful and satisfactory burning of highly caking low ash fusion bituminous coals.

A further object is to provide a Stoker grate which will result in successful burning of a wide range of sizes and qualities of bituminous coals regardless of their physical, chemical, caking or ash characteristics.

Another object is to provide a stoker grate bar formed of an inexpensive heat-resistant alloy having improved heat transfer and control of cooling air distribution whereby maintenance costs are reduced by reason of a longer operating life.

Another object is to provide a self-cleaning grate whic automatically cleans itself of ash or slag entering and sticking in the air openings thereof.

A further object is to provide a grate bar which resists deterioration in use thereby to maintain continuously high combustion eliiciency.

Still another object is to provide a Stoker grate wherein theArate of combustion may be readily controlled over the entire surface thereof thereby to eliminate substantially the usual problems of excessive smoke, clinkering and slagging.

In an extensive program of combustion research, it has been established that, in order to burn highly caking low ash fusion coal successfully on grates, a grate must be provided which will make possible:

(l) Control of coal distribution,

(2) Continuous ash discharge,

(3) Rapid heat transfer through the grate bars,

(4) Automatic cleaning of clinker from grate air openings,

Y (5) Control of combustion over the entire grate surface,

(6) Control of combustion air distribution,

(7) Ample cooling air where most needed.

These objects are accomplished by provision of a Stoker grate in accordance with the invention, which will now be described with reference to the accompanying drawing, in which Figure l is a partial plan view of a grate in accordance with the invention,

Figure 2 is a side elevation of a stationary grate bar of the invention,

Figure 3 is a side elevation of a moving grate bar of .the invention.

Athe nose portion of the grate, a wall y4l supporting a rocking bar 5 and defining the main air chamber 6 beneath the nose portion of the grate, and a wall 7 supporting the tail portion of the grate and Vdefining an auxiliary air chamber 8 beneath the grate. l The grate 1 comprises a plurality of stationary bars 9 and a plurality of alternating moving bars 10.

Each stationary bar 9 (as shown in Figures 2, 4 and 6) is of solid generally T-shaped cross-section and has a substructure web 11 and a widened top portion 12 providing a burning surface 13. Each bar 9 has a nose section 14 provided with a notch 15 in its under surface for reception of a flange 16 on the Wall 3. Each bar 9 also has a cross web 17 of approximately the same width as top portion 12 and having a curved under surface 18 for sliding engagement by the flange 19 of the rocking bar 5.v Each bar 9 also has a 'tail section 20 of relatively massive solid formation and supported upon wall 7 and an intermediate section located between the nose section and tail section.

Each moving bar 10 (see Figures 3, 5 and 7) is closely similar in structure to that bar 9 and has corresponding a substructure web 21, top portion 22, burning surface 23, nose section 24, cross web 25, and tail section 26. However, nose section 24 has a at under surface 27 for sliding engagement with wall 3, and cross web 25 has a notch 28 for receptionjof flange 19 of rocking bar 20 to impart movement tolthe bar 10.

Each` bar 9 and 10 has a plurality. of air. openings in the form of slots 29 in each edge surface of the top portions 12 and 22 and between the nose 'and'. tail sections thereof. Each slot is thus defined bya plurality of ribs 30 on each such edge surface. The slots are of approximately equal length but are progressively shallower from the nose to the tail section of the bar. Preferably, each slot is of greater length at the lower portion thereof than at the top portion thereof. For this purpose, each rib 30 has a tapered surface 31 (say, at an angle of 20) defining the rearward wall of the slot. The ribs 30 of one bar mesh with the ribs of the adjacent bars, as shown in Figure 1. While the length and depth of the slots (and the corresponding dimensions of the ribs) will vary in accordance with the desired size of air openings required for proper combustion of the particular fuel employed, a slot from 2 to 21/2 inches in length is satisfactory. for one example of installation. Again, by way of example, in the modification shown, the two slots adjacent the nose section maybe W16 inch in depth, the two slots adjacent the tail section 1/16 inch in depth, and the intermediate slots 1A; inch in depth. Generally speaking, each slot will be at least one inch in length and not greater than one-half inch in depth. It will be apparent that, in the example shown, there are seven slots provided in the side of each bar but this number will vary as required in each installation.

Right and left grate bars, one of which is indicated at 32 in Figure 1, are provided at each end of the grate. It will be understood that each such bar 32 will have complementary slots 29 and `ribs 30` on one side only, the other side being at to bearA against the end plates 2.

It will be apparent that the air openings as provided by the slots and intermeshing ribs willbe sizedto proportion the air required for proper combustion over the entire grate. At the nose, which is next to the retort, where the coal bed is thickest, the air slots are largest, and they are progressively reduced to nothing at the tail, where only ash remains. Y

The cross webs 17 and 25 match with each other4 over the whole length of the grate. These cross webs control combustion air by directing it from the main air chamber 6 to the nose sections of the bars, where the most combustion air is needed. Since the nose endalso requires the most cooling air, the cross webs serve a dual function.

Since the massive solid tail sections have no air openings therebetween, longer life is provided because the absence of air prevents burning on the tail surfaces. Thus, the tail sections of the bars are readily kept cool by the constantly changing air in the auxiliary air chamber 8.

Furthermore, the cross webs direct suliicient air to the noses of the bars to complete combustion at the center of the grate, leaving only ash at the tails.

The meshing of the ribs in the slots ensures that as the moving bars slide slowly back and forth, any ash or slag which nds its way into the air openings is forcibly removed.

ln burning highly caking low ash fusion bituminous coals, it is desirable that the percent of total air` openingsinthe` grate be in the rangeof 3% to 4% of the total burning surface. For. free burning or high ash fusion coals, the bars would be modified to reduce the total air openings to approximately 2% of the burning surface, and by sloping the slots to provide non-sifting air openings.

In accordance with a further feature of the invention, the grate bars described above are preferably cast in an inexpensive alloy which in field trails has shown considerable superioriity over grey iron in resistance to heat and oxidation growth with consequent increased grate life.

Examples of satisfactory grey iron alloys in accordance with the invention are given below:

m m% w% m% silicon.-. 2. 40-2. 6o 2. 00-2. 50 2. to-2. nu Sulphur 0.1;] max 0. max. 0.15 max. Phnspho 0. 30 max 0.30 max. 0. 3() max. Manganese.. O. 50-0. 70 0. 500. 70 0. 50-0. 70

2() Total Carbon.. 3.30-3.5O 3.30-3. 50 53.30-13.50 Nickel n. 90-1. 1o o. tio-1. 1o Chromium o. iso-1. 1. irl-1. 75 1. 8M. 20

We claim: 1. A grate bar of unitary cast form having a nose section an intermediate section and tail section and comprising a supporting web and a top portion havmg an upper burning surface, a lower surface contiguous with said web, and side edges joining said surfaces and disposed laterally outwardly of said web, each sald side edge having a plurality of ribs thereon, said ribs defining slots extending from said upper surface to said lower surface, each said slot having a longitudinal extent substantially greater than that of each said rib the endmost of said slots being located respectively in said nose section and in said intermediate section, said nose section Slot being of greater depth than said other endmost slot and the intermediate ones of said slots being of less depth than said nose section slot and of gradually decreasing depth from said nose section towards said tail section, said ribs on one of said side edges being offset with respect to said ribs on the other of said side edges whereby` said slots in one ofsaid side edges are in laterally disaligned relation with said slots in the other of said side edges.

2. A grate bar as defined in claim l, including a cross web between said nose section and said intermediate section and extending from said top portion to the edge ofsaid supporting web, said cross web having side` snrfaces eachV in substantially coplanar relation with one of side edges.

3. Al Stoker grate comprising a plurality of movable and stationary grate bars, each said bar being of unitary cast form and having a nose section, an intermediate section a tail section, a supporting web, and a top` portion having an upper burning surface and a lower surface contiguous with said web, each said side edge having a plurality of ribs thereon, said ribs defining slots extending from said upper surface to said lower surface, the endmost of said slots being located respectively in said nose section and in said intermediate section, said nose section slot being of greater depth than said other endmost slot and the intermediate ones of said slots being of less depth than said nose section slot and of gradually decreasing depth from said nose sec tion towards said tail section, said ribs on one of said side edges being offset with respect to said ribs on the other of said side edges whereby said slots in one of said side edges are in laterally disaligned relation with said slots in the other of said side edges` said ribs 0f one ofsaid bars extending into said slots of an adjoining one ofsaid bars, the longitudinal extent of cach said rib being less than that of each said slot to permit longitudinal reciprocation of'said movable bars rela* tively to said stationary bars and to provide a plurality 0f air openings extending through said-grate.

4. A stoker grate as dened in claim 3, each said having a percent of total air openings in the range of bar having a cross web between said nose section and 2% to 4% of the total burning surface. said intermediate section and extending from said top portion to the edge of said supportilg webl,1 said cross References Cega in the me of this patent web having side surfaces each in su stantia y coplanar 5 F relation with one of said side edges, the cross web of UNIED STATES PATENTS each said bar being in substantial abutment with the 527,593 CoXe Oct. 16, 1894 cross Webs 0I adjacent bars, said cross webs thereby 979,661 Fortune Dec. 27, 1910 constituting means for directing air to said nose sections. 1,433,180 Coplan Oct. 24, 1922 5. A Stoker grate as defined in claim 3, said grate 10 1,910,034 Mitchell et a1. May 23,1933 

